A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., including part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.
Prior to applying patterns to a wafer using a lithographic apparatus, the wafer generally needs to be measured and modelled so as to properly align the wafer and to correct wafer deformations during patterning. A critical performance measure of the lithographic process is overlay, the accuracy of alignment of features in two layers in a device (or between features formed by two patterning steps in the same layer). Alignment sensors having multiple color channels are used in the known lithographic apparatus, to try and obtain the best possible position measurements prior to patterning. These position measurements are used to calculate a substrate model for each wafer.
To improve overlay, additional measurements are made of performance on prior substrates that have been patterned, to identify and correct deviations introduced in the patterning step and/or other steps. Various tools for making such measurements are known, including scanning electron microscopes, which are used to measure performance parameters such as overlay.